This invention relates to fume hoods and, in particular, to a fume hood which is provided with an improved airflow system. Fume hoods are commonly used in the laboratory for providing a ventilated work area for laboratory activities involving hazardous materials, generated fumes, aerosols, gases and particulate matter. The fume hood protects laboratory personnel and equipment by confining, containing and exhausting these materials.
Conventional laboratory fume hoods include an exhaust blower for removing air from the fume hood, and thus draw their air supply completely from the laboratory room. Since these hoods withdraw conditioned air from the laboratory, a considerable amount of expensively heated conditioned air is lost through this type of hood.
Auxiliary air fume hoods are designed to reduce the amount of conditioned air which is consumed by supplying untempered, non-conditioned auxiliary air to the fume hood. Hoods equipped with auxiliary air fans and related duct work derive approximately 70% of their air supply requirements from outside the laboratory building. The auxiliary air which is introduced into the hood and any conditioned air which is drawn into the fume hood from the laboratory must sweep across the entire work surface to prevent the escape of fumes from the hood. In addition, when the sash is closed, the fume hood must be provided with a means for preventing excessive velocities of air at the face of the hood. High face velocities and excessive turbulence are undesirable because they jeopardize the safety of the operator and interfere with normal operations carried out in the work area.
The flow of auxiliary air supply at the face of the hood is affected by the temperature of auxiliary air in comparison to the temperature of the room air. When auxiliary air is supplied at relatively high temperature with respect to room temperature, a significant amount of the auxiliary air drifts into the laboratory room. It is desirable to draw as much as possible of the auxiliary air to the exhaust system so that untempered air does not mix with or dilute room air and thereby interfere with attempts to provide a comfortable work environment. Heretofore, fume hoods having auxiliary air supply systems have not been able to fulfill the exhaust requirements of fume hoods without undesirably increasing the amount of conditioned air which is drawn into these hoods. In addition, these fume hoods have not had provisions to capture a significant portion of the auxiliary air when the temperature of the auxiliary air is greater than that of the room air.